System for extracting heat from a liquid metal target



July 1, 1969 SYSTEM R. I. HODGE Filed June 10. 1966 FOR EXTRACTING HEAT FROM A LIQUID METAL TARGET United States Patent O 3,453,175 SYSTEM FOR EXTRACTING HEAT FROM A LIQUID METAL TARGET Ronald I. Hodge, 16 Laurier Ave., Deep River, Ontario, Canada Filed June 10, 1966, Ser. No. 556,656 Int. Cl. G21c 15/00; H015 33/00 U.S. Cl. 176-11 9 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to an intense neutron generator having a metal target irradiated by a proton beam from an accelerator having a beam current for example of up to 100 milliamperes and energies of up to l gev. In order to provide a target for generating neutrons and also to remove the heat generated by irradiation it has been proposed to use a liquid metal such as a lead-bismuth eutectic alloy llowing in a heat exchange loop, with the generated heat being extracted through a heat exchanger. The total power of such a proton beam may be, for example, of the order of 60 megawatts, of which approximately 35 megawaltts is converted to heat when the proton beam is projected against the metal target.

One of the problems involved with the generation of neutrons by this process is that a beam of protons must rst be generated and then projected down an evacuated beam tube to the liquid metal target. The high enery protons used to irradiate the target are generated in apparatus which is evacuated to a vacuum of about l6 torr. Thus it is necessary to maintain the integrity of the vacuum in the beam tube. One possible solution to this problem would be to provide a vacuum tight window made of a material transparent to protons. Unfortunately no known material is suiciently transparent to avoid internal heating and radiation damage when used to seal a proton beam tube.

The present invention overcomes these problems by providing a liquid metal target for the proton beam and at the same time providing a vacuum seal for the beam tube, without the use of a window at the end of the beam tube.

In accordance with the present invention, the beam Ltube is introduced into the vapour space of what is in effect a Torricellian barometer. Preferably a vacuum pump is connected to the vapour space to maintain a predetermined vacuum. This removes in-leaking air and volatile contaminants generated in the liquid metal.

In general liquid is introduced into a vertical tube at an intermediate level and flows down the tube to a sink at pressure ps. Above the entry level a vacuum is maintained, i.e. a space in which the pressure is pa-0- In the ow length there .are frictional losses Ap. Two conditions must be met:

(i) The vapour pressure of the liquid must nowhere exceed the local static pressure, and particularly at entry must not exceed the desired vacuum pressure pa.

3,453,175 Patented July l, 1969 ICC (ii) The entry must be at a height L above the sink such that hiel-MH@ where p is the mean density of the liquid, and g is the local gravitational acceleration.

ln the preferred arrangements, the liquid is introduced as a film on the upper surface of the vertical tu-be and makes in effect its entrance by ilooding `the tube at the height L described above.

If the liquid entrance were Ilower than L, the liquid surface would rise to L and the entrance would then be submerged. There would be no through flow in that part of the liquid above the entrance. Such an arrangement might be suitable in low power applications.

In accordance with one aspect of the invention a leadbismuth .alloy is introduced over a weir and allowed to run down the sides of the target tube until it reaches the target level, and the proton beam is thus allowed to penetrate the moderator tank before striking the liquid metal target. Above the target, the centre of the tube is free of liquid metal, for the proton beam and this space may be evacuated to the same vacuum as the rest of the beam tube. The flow may be easily and stably attached to the walls by inlet swirl, i.e. by introducing the liquid metal as a whirlpool. The vapour pressure of a llead-bismuth eutectic alloy .at about 250 C. is lower than the internal pressure in known proton accelerators so that the evacuated zone is not rapidly contaminated by lead-bismuth vapour.

In the drawing which illustrates embodiments of the invention;

FIGURE 1 shows a schematic diagram of a lead-bismuth circulating circuit in accordance with the present invention in which the vacuum in the beam tube is preserved, and

FIGURE 2 shows an alternative target tube entry section.

As shown in FIGURE 1, a beam tube 10 is provided down which a stream of protons is guided to strike the liquid metal target at 12, The target is contained within the target tube 16 .and surrounded by a moderator 13 such as heavy Water. Assorted beam holes are provided through the moderator in accordance with known practice so that streams of neutrons will be obtained for experimental purposes. The liquid lead-bismuth in the target tube 16 flows at 14 .away from the `target area 12 to a heat exchanger 15, where the heat generated by the proton beam striking the liquid metal target is extracted. The pump 17 is provided to circulate the leadbismuth eutectic around the heat exchange loop and the vacuum pump 18 is provided to maintain a suitable vacuum at the open end of the beam tube 10. The pump 17 is driven by a motor 19 and is connected to conduits 20 and 21 as well as Ito the level control tank 22, and the dump tank 23.

The operation of the apparatus of FIGURE 1 can best be understood by assuming that the circuit is empty and all of the lead-bismuth eutectic is in the dump tank 23. The lead-bismuth can be pumped from the dump tank 23 via the valve 24 to fill the system by pressurizing the dump tank 23 with nitrogen or any other suitable inert gas fed through the line 25 and the valve 26. With the system properly lilled, the level of the lead-bismuth liquid can be raised to the point L3. This level in the level control tank 22 and in the pump 17 may then be maintained by control of the pressure of inert gas such as nitrogen fed from the line 27, via the valve 28 into the level control tank 22, and via Ithe passage 29 into the pump 17, excess nitrogen suitably being released from the level-control tank 22 via the line 30 and the valve 31. The level of the liquid in the rest of the system can then be increased by applying further pressure on line 25 until the level rises to the level L5. The level L is then the reference lling level for the .system before circulation of the liquid begins. The pump 17 is started and the liquid now moves to the levels L4 and Ls by virtue of the flow of the liquid about the circulating loop. The vacuum pump 18 is used to maintain the vacuum in the space above the liquid ,and in the beam tube 10.

The level L6 at which the surface 12 is located is determined by the pressure at the level L3 in the level control tank, the pressure at the surface 12 (a vacuum), and the vertical difference in elevation between L3 and L6 as well as the frictional loss 0f head from L6 to L3 when the liquid flows through the loop.

The pressure at the pump inlet must not fall below a value at which cavi'tation occurs. For example, a value of 5 feet of lead-bismuth has been found satisfactory elsewhere. The hydrostatic head Lb-Ll must exceed this value by at -least an amount equal to the hydraulic losses in the conduit 21. L6 is fixed in practice by the fact that it must coincide with the elevation chosen for the top of the target. Therefore L1, the pump elevation must be chosen to give the correct value for L-L1. Once the system is built, L-Ll can be .adjusted either by using an adjustable resistance in the pipe from Ls to L1 to vary the hydraulic loss of conduit 21, or lby varying the pressure in the level control tank.

The wall thickness of the target tube 16 may be chosen to resist collapse in the section which has zero internal pressure and a pressure approximately one atmosphere on the outer wall. This occurs at the upper end of the tube 16 adjacent the weir 32 in FIGURE l. At the lower end of the target tube 16 the reverse situation occurs, where the internal pressure is highest, giving a low value of tensile stress in this region.

A small amount of whirl will produce a ow attached to the tube wall for the case of a well shaped weir. For the most severe possible case-that of the right-angled weir, produced by a constant diameter target tube, attached to the bottom of a tank, stable and attached ow is achieved by whirl angles of 45 or more. The whirl angle is measured between the flow direction and a radius at the lip of the Weir.

The following is an example of a typical system in accordance with the present invention wherein the conduits 20 and 21 of the liquid lead-bismuth loop are 8 inch inside diameter lines. The flow is about 3.3 cubic feet per second, and the temperature difference is about 250 C. The hydraulic loss in conduit 21 is calculated to be equivalent to about 8 feet of lead-bismuth. Therefore the level L5 will be approximately 13 feet above the level L1. Current layouts indicate that a value of Le-Ll of about 18 feet is necessary to allow installation of the equipment.

FIGURE 2 shows an alternative form of target tube entry section, the circular weir 32 of FIGURE 1 has been replaced with an annular section 33 equipped with vanes 34 followed by an annular nozzle 35. The lead-bismuth eutectic emerges from the annular nozzle 35 as a falling lilm which adheres to the walls of the target tube 16. As before the vacuum pump 18 may be connected to the target tube above the surface 12. This form of the invention has the advantage that shielding is simplified and that the system possess a reduced overall diameter.

Accordingly the present invention provides a practicable system for providing a liquid lead-bismuth target for an intense neutron generator which provides the necessary heat exchange loop for removing the heat due to the proton beam impinging against the lead-bismuth target and at the same time provides simple and reliable means for maintaining the vacuum integrity of the proton acceleration system without the use of a proton window.

I claim:

1. A System for extracting heat from a liquid metal target against a surface of which a stream of high energy particles impinges and for maintaining a vacuum adjacent said surface comprising a closed loop around which said liquid is adapted to be circulated, said loop including a heat exchanger, and a target tube, said liquid metal partially filling said target tube to a predetermined level, a vacuum pump connected to the liquid-free space in said target tube, and a beam tube projecting into said target tube such that when liquid ilows through said loop the vacuum pump is in direct communication with the end of the beam tube and maintains a predetermined vacuum in the proton accelerating system.

2. A system according to claim 1 wherein a weir is included in said closed loop over which said liquid ows.

3. A system according to claim 1 wherein an annular nozzle is included in said closed loop through which said liquid flows.

4. A system according to claim 1 including a dump tank into which all of the liquid lead-bismuth in the system may be dumped.

5. A system according to claim 1 including a levelcontrol tank coupled to said pump and having an inlet at the upper part of said level-control tank for the admission of an inert gas, to control the height of liquid in the target tube.

6. A system according to claim 2, wherein said weir is circular and said liquid flows over said weir toward the centre thereof.

7. A system according to claim 3 wherein a plurality of vanes are provided in said closed loop above said nozzle to cause rotation of said liquid about said beam tube.

8. A system according to claim 4 including a source of inert gas under pressure connected to said dump tank whereby said liquid may be forced out of said dump tank to ill said system.

9. A system according to claim 6 wherein means are provided to cause said liquid to swirl over said weir.

References Cited UNITED STATES PATENTS ARCHIE R. BORCHELT, Primary Examiner.

U.S. Cl. X.R. Z50-84.5

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,453,175 July 1 1969 Ronald I. Hodge It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

In the heading to the printed specification, after line 5, insert assignor to Atomic Energy of Canada Limited, Ottawa, Ontario, Canada, a corporation of Canada Signed and sealed this 16th day of June 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer WILLIAM E. SCHUYLER, JR. 

